1. Field of the Invention
The present invention relates to a piezoelectric transformer used for various high voltage power supplies, and more particularly to a piezoelectric transformer for a small-sized high voltage power supply that is required to be miniaturized and to have a high reliability.
2. Description of Related Art
In most of cases, high voltage generating transformers used for a photocopiers and a backlight of crystal liquid displays have been conventionally composed of electromagnetic solenoid transformers.
On the one hand, a piezoelectric transformer having an operation principle completely different from that of the electromagnetic solenoid transformer has been proposed. A typical example of the conventional piezoelectric transformer is constituted of an elongated piezoelectric plate and a pair of driving electrodes formed on upper and lower surfaces of a half portion of the piezoelectric plate in a lengthwise direction, respectively. An input AC (alternating current) voltage is applied between the pair of driving electrodes. Therefore, a portion of the piezoelectric plate applied with the input voltage is called a "driver section". The piezoelectric plate in the "driver section " is polarized in a thickness direction. The other part of the piezoelectric plate is called a "generator section", and the piezoelectric plate in the "generator section" is polarized in a lengthwise or longitudinal direction.
An output electrode is formed on an end surface of the generator section of the piezoelectric plate, so that an output voltage is obtained between the output electrode and one of the driving electrodes. For fixing the piezoelectric transformer, a support member is positioned at a node in a lengthwise direction of a longitudinal mechanical oscillation in length.
In this condition, if an AC voltage having a resonance frequency of the longitudinal oscillation in a lengthwise direction of the piezoelectric plate is applied between the pair of driving electrodes, the piezoelectric plate resonates mechanically. As a result, the high voltage having the same frequency as that of the input voltage is outputted from the "generator part".
In the above mentioned conventional piezoelectric transformer the connection to the electrodes of the driver section and the generator section is not effected at any node of the oscillation, which lowers the reliability remarkably. In order to resolve the above mentioned disadvantages, an improved piezoelectric transformer has been proposed.
This improved piezoelectric transformer is composed of an elongated piezoelectric plate divided into a pair of driver sections and a generator section between the driver sections. Each of the driver sections includes a pair of driving electrodes formed on upper and lower surfaces of the driver section, and the generator section includes an output electrode extending on the piezoelectric plate in its transverse direction at a position equally having the generator section of the piezoelectric plate. This output electrode also functions as a support member.
The pair of "driver sections" are of the piezoelectric plate are polarized in the same thickness direction, for example in an upward direction. The two halves of the piezoelectric plate of the generator section are polarized in directions opposite to each other in a lengthwise or longitudinal direction, for example, in directions towards the output electrodes.
With this construction, an input AC voltage is applied between the pair of driving electrodes of each of the driver sections, and an output voltage is obtained between the output electrode and one of the driving electrodes of each of the driver sections.
The two above mentioned conventional piezoelectric transformers are small-sized and can generate a high voltage. Accordingly, these transformers are very attractive to an inverter used for back-lighting a liquid crystal, which is strongly required to be miniaturized. However, even the second mentioned improved piezoelectric transformers has large problems as follows:
(1) The driver sections of the two ends of the piezoelectric plate are driven in the same phase. Therefore, in the case that the piezoelectric transformer is driven using a DC power source, the change with time of a load for the power source is very large.
(2) The input voltage must be relatively large, so that the degree of freedom for selection is limited.
(3) In connection to the output voltage, the degree of freedom for the selection of the transformation ratio is also small.
(4) Since there are only a small number of the terminals and support points, the oscillation impact risks to cause a damage in a using condition.